Magnesium-based thin films and nanostructures are a subject of extensive research as they are becoming increasingly more utilized for optical hydrogen sensing, switchable mirrors and solar absorbers, and as model alloys for designing and understanding bulk hydrogen storage materials.
Magnesium and magnesium oxide are known to have poor activity towards hydrogen dissociation, which is the first step in the absorption process. Because of this, Pd catalyst films are normally deposited on the fresh magnesium surfaces to aid the sorption kinetics. Increasingly, these catalysts consist of bi-layers, consisting of Pd on an oxide or a metallic support. This intermediate layer serves the critical role of reducing the highly deleterious interdiffusion between the Pd and the underlying hydrogen storing material. Bi-layer catalysts may also exhibit enhanced kinetics due to strong metal-support interactions (SMSI), though this effect is less explored for two metals as it is for metals on oxide supports.
Remhof et al. were the first to utilize a metallic (Nb) intermediate layer between the catalytically active palladium cap and the hydrogen storing yttrium phase, though not providing a comparison of the sorption kinetics without it. An early work on oxide-based intermediate layers for hydrogen storing materials was by researchers who examined nano-scale Y2O3 buffer layers for YHx, and nano-scale AlOx buffer layers for LaHx and YHx. In both studies the authors concluded that the presence of an intermediate oxide layer did not impede hydrogen loading. Rather these purposely grown buffer layers had a beneficial role of impeding the interdiffusion of the active base metal and the Pd catalytic cap. The interdiffusion would presumably result in the formation of a discontinuous layer of binary intermetallics, the oxidation of the underlying active metal, and a subsequent loss of hydrogen dissociation catalytic activity. Subsequent studies utilized Ti or Fe underlayers to reduce the interdiffusion of the palladium with the base material and the consequent formation of intermetallics. The base materials tested include pure Mg, a range of Mg—Ni alloys, Y and Mg—Al alloys.
MgAl alloys have attracted interest for their favorable hydrogen storage properties for more than 20 yrs. This relatively simple system, available commercially in ingot form, is attractive due to a combination of the relative low material cost and the environmentally benign nature of the alloy. In general, most studies found MgAl alloys promising; however, the kinetics was still inadequate for the rapid low temperature desorption required of a commercially viable hydrogen storage material for automotive and portable hydrogen applications. Low temperature hydrogen absorption in MgAl thin films has been achieved with a single layer Pd, a bilayer Pd/Ti, and Pd/Fe(Ti) catalysts. However, appreciable hydrogen desorption is only possible at temperatures too high for practical applications.
Binary Mg—Fe and Mg—Ti alloys are a subject of extensive research since they possess significantly accelerated kinetics relative to other Mg-based systems. Both systems show good gravimetric and volumetric hydrogen densities that vary with the alloy content but can be equivalent to or even higher than that of pure Mg. At equilibrium, neither Fe nor Ti has appreciable solubility in Mg, nor do they form any intermediate phases. Upon hydriding Mg—Fe system forms a combination of Mg2FeH6 and MgH2, the ratio of the two phases depending on the composition. The pure Fe phase does not form a hydride itself. Mg2FeH6 begins to desorb at an equivalent temperature as MgH2, about 300° C., and has a similar heat of formation (actual reported values vary). In the Mg—Ti alloys the hydrided structure is poorly understood. It appears to be more complex than simply a mixture of the equilibrium MgH2 and TiH2 phases. This is supported by the known stability of binary TiH2, which has a heat of formation of −136 kJ/mol and therefore should not desorb at 300° C.
Though the binary Mg—Fe and Mg—Ti bulk alloys and thin films are fairly well studied, the ternary Mg—Fe—Ti system has not received the same level of attention for hydrogen-related applications.
Researchers have reported significantly accelerated kinetics in binary Mg—Fe and Mg—V systems relative to other Mg-based alloys. Both Mg—Fe and Mg—V show good gravimetric and volumetric hydrogen densities that vary with the alloy content. At equilibrium, neither Fe nor V has appreciable solubility in Mg, nor do they form any intermediate phases. Upon hydriding Mg—Fe system forms a combination of Mg2FeH6 and MgH2, the ratio of the two phases depending on the composition and synthesis method. The pure Fe phase does not form a hydride itself Mg2FeH6 has a similar heat of formation as MgH2 (reports vary from 70-80 kJ/mol). Due to the need for Fe diffusion, the sorption cycling kinetics of Mg2FeH6 are relatively slow. Even under rough vacuum neither Mg2FeH6 nor MgH2 normally show appreciable desorption below 300° C.
Mg—V powder composites display some of the fastest hydrogen sorption kinetics of any magnesium-based system. The heat of formation for the most commonly reported form of vanadium hydride VH0.5 is −35 to −42 kJ/mol H. Thus one would not expect this phase to be stable at the hydrogenation temperatures/pressures utilized for magnesium. Authors did report the presence of VH0.81 phase in the hydrogenated Mg—V powders, deduced from x-ray analysis. However the plateau pressure-composition-temperature (PCT) data for the composite was identical to that of α-MgH2.
Binary Mg—Fe and Mg—V bulk alloys (powders) and Mg—Fe thin films are relatively well studied for hydrogen-related applications. However the ternary Mg—Fe—V system, be it in bulk or thin film form, has received little attention. Vanadium is expensive. A more economical bi-metallic catalyst that is comparable or even surpasses vanadium in its performance would be highly sought after.